The present invention relates to a process for preparing printing plates, especially for those useful for printing on corrugated boards.
The process of the invention is useful for plates for printing not only on corrugated boards but also for usual paper materials.
Preparation of printing plates usually requires a complex process including many steps as seen in FIG. 7. The conventional process will be described with reference to the case where for a better understanding, the characters "F" and "L" roughly sketched as at 1 are to be printed in different colors.
Step a
The characters are drawn on paper in specified colors to prepare originals 2 for printing plates.
Step b
The originals 2 are photographed by a letterpress camera on an enlarged or reduced scale corresponding to the print 6 to be made, to obtain negative films 3.
The letterpress camera is provided with a front mirror and adapted to photograph reflected images. The image of the original, which is reflected from the mirror and passes through the lens, is projected onto the coating of the film as an inverted normal image having the original right-to-left relationship. Accordingly when the negative film 3 is turned through 180 degrees, the image as seen from the film substrate side is an erect reverse image as seen in FIG. 7(b).
It is to be noted that in the letterpress printing process, the term "normal image" or "reverse image" refers to the image as seen through the film substrate, with the photosensitive coating side of the film positioned outward and does not refer to the image as seen directly on the coated surface of the film. Thus, the reverse image on the negative 3 is a normal image when it is viewed directly on the coated surface.
Step c
Each negative film 3 is placed on the coating 42 of photosensitive paper 41 with the film substrate in intimate contact therewith and with the coated film surface 31 out ashown in FIG. 5, and the paper is exposed to light to obtain a print 4 bearing the image as a positive normal image.
Step d
The print 4 is cut to a piece of paper 5 having a suitable size and bearing the image. The pieces 5 are arranged on a mounting board 7 of the same size as the print 6 to be obtained, the surface of the board 7 is cut away in the same size as the pieces 5, and the pieces 5 are fitted into the mounting board 7, with the surfaces of the pieces 5 flush with the surface of the mounting board 7.
The layout obtained is identical with the printed layout on the print 6 to be finished with the exception of color.
Step e
The mounting board 7 with the layout of characters is photographed again with the letterpress camera to prepare identical negative films 8 bearing reverse images only and identical in number to the number of colors.
Step f
Only one image on one of the negative films 8 is obliterated with an opaque ink, while only the other image on the other negative film 8 is similarly inked out for color separation to obtain negative films 9 for the different colors.
Step g
Each of the negatives 9 is placed on a fresh film with the coated surface of the former in contact with the photosensitive surface of the latter, followed by exposure and printing to obtain a color separation film 10 bearing the image in the form of a positive normal image for the given color.
Step h
A plate-making color separation negative 12 is prepared from each film 10 using an enlarger 23 and a fresh film. When the printing plate 11 finally obtained is fitted around a printing roller for printing, an enlarged image will be obtained, so that the negative 12 is such that the image thereon is formed as contracted in the direction of rotation of the printing roller by an amount corresponding to the enlargement.
FIG. 6 shows the enlarger 23, in which the color separation film 10 is moved horizontally with the coated surface out, while the fresh film is fitted around a drum 24 and rotated at the same time. The color separation film 10 is exposed to light from above to form an image on the fresh film on the drum 24 through a mirror 25, lens 26 and slit 27 and obtain a print.
Since the peripheral speed of the drum 24 is slightly lower than the horizontal feed speed of the color separation film 10, the image of the film 10 is formed on the film over the drum 24 as a reverse image slightly contracted in the direction of rotation of the drum, whereby the plate-making color separation negative 12 is obtained.
Step i
A photosensitive flexible synthetic resin sheet (for forming a flexible relief printing plate) is exposed to light in intimate contact with each negative 12, whereby the exposed surface area of the resin sheet is chemically reacted and made insoluble. The unexposed area is etched away by a known chemical treatment to obtain a printing plate 11 having a reverse image in relief.
Each flexible printing plate 11 thus repared is set on a printing roller to print the image on paper 6 in color by letterpress printing.
Because the positive-negative relationship of the film, as well as of the photosensitive paper, is dependent on the kind of the photosensitive agent forming the coating, the foregoing description is merely exemplary of the conventional process. It is of course possible to print the image of a negative on photosensitive paper or film in intimate contact therewith to obtain a negative image.
The photosensitive agent coating the film and plate-making material may be such that it become soluble when exposed. In this case, it is possible to photograph an original of positive image to obtain a film bearing a positive image (positive-positive film).
The normal image-reverse image relationship is also not limitative; it is possible to photograph a normal image with a mirror- or prism-incorporating camera to obtain a normal image by reversing the image twice through the lens and prism.
Various plate making proceses have been practiced which resort to combinations of positive image-negative image relationship and normal image-positive image relationship.
The conventional process described above requires many steps for preparing finished prints and necessitates manual work for the step d of laying out the paper pieces 5 and fitting the pieces 5 into surface cutouts in the mounting board 7 and for the image inking-out and color separation step f, so that it has been thought impossible to automatically produce printing plates in large quantities.
Furthermore, the negative films 3, 8, photosensitive paper 4, mounting board 7 and color separation film 10 prepared are used only once and are not reusable. This renders the printing plate expensive to make.
While a sample of finished print must be approved of by the customer who placed an order for prints with the rough sketch 1, the plate maker shows the customer a print of the negative films 8 on sensitive paper obtained by contact printing or the mounting board layout 7 for this purpose. Accordingly in the event of a change in the customer's instructions, the same steps must be repeated again to comply with the customer's request. Further because the sample, i.e. the mounting board 7 or the print, bears positive images only, the conventional process involves the inconvenience that no color image sample is available before the process is executed to the final step (step i).